Photothermal therapy (PTT) and photo-dynamic therapy (PDT) have emerged as effective approaches for cancer treatment. Herein, we present atomic-level scale (0.5 nm thickness) ultrathin sulfur-doped molybdenum oxide nanorings (S-MoOx A-NRs) and with surface coating of polyethylene glycol (PEG) (PEG@S-MoOx A-NRs). This nanomaterial shows high absorbance in the near-infrared (NIR) range and can be used as a sensitive photoacoustic imaging (PAI) contrast agent. Upon NIR irradiation, the particles show high photothermal conversion and reactive oxygen species (ROS) generation, which effectively kills cancer cells both in vitro and in vivo. The PEG@S-MoOx A-NRs allow PAI and synergistic PTT/PDT therapy, which is demonstrated as a promising theranostic strategy for future cancer therapy.